Field of the Invention
The invention relates to an optical smoke alarm with a detector unit that works according to the scattered light principle.
Smoke alarms of this type are also known as fire alarms. They typically contain a housing with at least one smoke-inlet opening and the detector unit accommodated in the housing for the smoke detection. The detector unit preferably contains an optical measuring chamber, which is shielded against ambient light, but permeable to smoke to be detected. The latter typically also contains a plurality of louvers to shield ambient light and is therefore also called a labyrinth.
The detector unit in question contains a light-emitting diode to irradiate particles to be detected and a photo sensor which is spectrally sensitive thereto to detect the light scattered by the particles. Typically an electronic control unit is also connected to the light-emitting diode and the photo sensor as part of the smoke alarm. The control unit is configured to output a warning and/or an alarm if a respective minimum concentration value of smoke can be detected.
Published, European patent application EP 0 877 345 A2, corresponding to U.S. Pat. No. 6,011,478, discloses a scattered-light smoke alarm containing a light-emission device to emit light with two different wavelengths, a light-receiving device to receive scattered light with the two different wavelengths, a calculating device for performing a calculation necessary for the smoke detection from a scattered light output of the first wavelength and a scattered light output of the second wavelength of the light-receiving device and a smoke-detection processing device for performing a smoke-detection process on the basis of a calculation result output from the calculating device. According to an embodiment therein according to FIG. 13, the use of a light-emission device, i.e. a two-color light-emitting diode is known, in which a first and second light-emission device are arranged at positions very close to one another and emit the light of the first and second wavelengths in the same light-emission direction.
Also known from the prior art are detector units that use two differently colored light-emitting diodes and a photo sensor in one or two scattered-light arrangements. It is known to use a red-luminous LED or an infrared LED to emit the red or infrared light and to use a blue or violet-luminous LED to emit blue or violet light. By means of a suitable evaluation of the respective colored scattered light received from the photo sensor, it is then possible to perform an evaluation with respect to the particle size of the smoke particles detected. A suitable assessment of the particle sizes detected enables, for example, a differentiation to be made between smoke, dust and steam. This enables the output of a possible false alarm to be prevented.
The known smoke alarms are typically configured for operation in a line of alarms with a plurality of further smoke alarms connected thereto or for battery-supported stand-alone operation. This means in both cases that only a very small average electrical power of less than 10 mW is available. The control of the light-emitting diodes for the emission of the respective light is, therefore, typically pulsed. Similarly, the entire “electronics” are adapted for the lowest possible power consumption.